Relationship between postoperative biomarkers of neuronal injury and postoperative cognitive dysfunction: A meta-analysis

Early biomarkers are needed to identify patients at risk of developing postoperative cognitive dysfunction (POCD). Our objective was to determine neuronal injury-related biomarkers with predictive values for this condition. Six biomarkers (S100β, neuron-specific enolase [NSE], amyloid beta [Aβ], tau, neurofilament light chain, and glial fibrillary acidic protein) were evaluated. According to the first postoperative sampling time, observational studies showed that S100β was significantly higher in patients with POCD than in those without POCD (standardized mean difference [SMD]: 6.92, 95% confidence interval [CI]: 4.44−9.41). The randomized controlled trial (RCT) showed that S100β (SMD: 37.31, 95% CI: 30.97−43.64) and NSE (SMD: 3.50, 95% CI: 2.71−4.28) in the POCD group were significantly higher than in the non-POCD group. The pooled data of observational studies by postoperative sampling time showed significantly higher levels of the following biomarkers in the POCD groups than in the control groups: S100β levels at 1 hour (SMD: 1.35, 95% CI: 0.07−2.64), 2 days (SMD: 27.97, 95% CI: 25.01−30.94), and 9 days (SMD: 6.41, 95% CI: 5.64−7.19); NSE levels at 1 hour (SMD: 0.92, 95% CI: 0.25−1.60), 6 hours (SMD: 0.79, 95% CI: 0.12−1.45), and 24 hours (SMD: 0.84, 95% CI: 0.38−1.29); and Aβ levels at 24 hours (SMD: 2.30, 95% CI: 1.54−3.06), 2 days (SMD: 2.30, 95% CI: 1.83−2.78), and 9 days (SMD: 2.76, 95% CI: 2.25−3.26). The pooled data of the RCT showed that the following biomarkers were significantly higher in POCD patients than in non-POCD patients: S100β levels at 2 days (SMD: 37.31, 95% CI: 30.97−43.64) and 9 days (SMD: 126.37, 95% CI: 104.97−147.76) and NSE levels at 2 days (SMD: 3.50, 95% CI: 2.71−4.28) and 9 days (SMD: 8.53, 95% CI: 7.00−10.06). High postoperative levels of S100β, NSE, and Aβ may predict POCD. The relationship between these biomarkers and POCD may be affected by sampling time.


Introduction
Postoperative cognitive dysfunction (POCD) affects direction, attention, consciousness, perception, and judgment of patients under general anesthesia [1], with an incidence of 8.9-46.1% [2]. Several factors, including neuroinflammation, oxidative stress, autophagy disorder, neuronal injury, and lack of neurotrophic support are hypothesized to contribute to POCD [3,4]. However, the specific pathogenesis of POCD remains unclear. Currently, the diagnosis of POCD is based mainly on neuropsychological tests. However, the composition of the test batteries varies greatly [5] and they are easily affected by factors such as culture, educational level, and language [6], which leads to their limited value for diagnostic application. Recent studies have found that anesthesia and surgery can cause neuronal damage and related biomarkers may be associated with postoperative cognitive outcomes [7,8]. Identifying the relationship between neuronal injury biomarkers and POCD may present another method of diagnosing POCD.
Several biomarkers have been confirmed to be associated with neurodegenerative diseases. Amyloid beta (Aβ) and tau proteins are included in the diagnostic guidelines for Alzheimer's disease (AD) [9]. Neurofilament light (NFL) is associated with the pathophysiology of amyotrophic lateral sclerosis, Guillain-Barré syndrome, Parkinson's disease, and AD [10][11][12][13]. S100β, neuron-specific enolase (NSE), and glial fibrillary acidic protein (GFAP) have been reported to be associated with the pathophysiology of stroke and traumatic brain injury [14][15][16][17][18]. However, the relationship between these biomarkers and POCD remains unclear. Therefore, we performed a meta-analysis of the relationship between neuronal damage biomarkers and POCD to explore whether these biomarkers have predictive value for this condition. The results of our analysis showed that the three biomarkers, S100β, NSE, and Aβ, have predictive value for POCD.

Sources and search strategy
We conducted a systematic literature review using the PubMed, Embase, and Cochrane databases to identify studies published up to April 2022. To bring together meaningful results, we focused more on neuronal damage markers that have been extensively studied [15,[19][20][21][22]. The search terms included the following keywords and their combinations: ("S100β" OR "neuron-specific enolase" OR "amyloid beta" OR "tau" OR "neurofilament light" OR "glial fibrillary acidic protein" OR "neuronal injury") AND ("postoperative cognitive dysfunction").

Study selection
To be included, studies had to meet the following criteria: (1) conducted in humans; (2) assessment of at least one specific neuronal marker in blood or cerebrospinal fluid (CSF); and (3) written in English. Studies with the following characteristics were excluded: (1) no clear differentiation between POCD subtypes, (2) lack of comparison groups, (3) non-human studies, and (4) non-English articles. Two reviewers (X.W. and X.C.) independently evaluated the retrieved articles according to the title, summary, or full text, and a third independent reviewer (W.C.) resolved discrepancies.

Data extraction and synthesis
Data extraction, synthesis, and a risk of bias analysis were guided by Preferred Reporting Items for Systematic Reviews and Meta-Analysis (S1 Appendix). Using standardized extraction tables, we recorded the following data: (1) authors and publication year, (2) research design, (3) sample size and participant characteristics, (4) type of surgery, (5) type of anesthesia, (6) biomarkers and sampling time and source, and (7) methods to diagnose POCD. For studies reporting only the median, range, or interquartile range, we applied the appropriate formulas to convert them to mean and standard deviation [23][24][25]. The data used for all analyses can be seen in S2 Appendix.

Assessment of study quality
Risk of bias was assessed for the individual randomized controlled trial (RCT) according to the Cochrane Collaboration Tool, which evaluated trials based on the presence or absence of randomization sequence generation, allocation concealment, selective reporting, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, and other forms of bias [26]. The quality of observational articles was independently classified using the Newcastle-Ottawa Scale [27]. The Newcastle-Ottawa Scale score ranges from 0 to 9 stars. A quality score was calculated based on three major components: (1) selection of study groups (0-4 stars), (2) comparability of study groups (0-2 stars), and (3) determination of the exposure and outcome of interest in case-control and cohort studies (0-3 stars). Studies with scores of 7 to 9, 4 to 6, and 0 to 3 stars were considered high, medium, and low quality, respectively. Disagreements were resolved by discussion and consensus.

Statistical analysis
If two or more studies reported factors of interest, the standardized mean difference (SMD) and the 95% confidence interval (CI) of each result were calculated using the random-effects model to evaluate the association between markers and POCD. The chi-square test was used to calculate the Q-value to quantify the heterogeneity between joint tests, and the I 2 index was used to determine the impact of heterogeneity in inconsistent calculations. The significance level of the heterogeneity P-value was established at 0.1, and the I 2 statistic was interpreted as follows: 0-40%, low; 30-60%, moderate; 50-90%, substantial; and 75-100%, considerable heterogeneity. Statistically significant heterogeneity was considered present at P <0.1 and I 2 >50%. Because the sampling time of each study varied greatly, pooled analyses were performed for different time points with more than two studies. Sensitivity analysis was used to test the impact of each study on the overall estimation. The funnel plot asymmetry test was recommended only when at least 10 studies were included in the meta-analysis. Therefore, we used Egger's test to assess asymmetry and publication bias. Statistical significance was established at P <0.05. The meta-analysis was carried out using Review Manager 5.4.1 (Cochrane Org., London, UK), and publication bias was evaluated using Stata 16 (StataCorp LLC., College Station, TX, USA).

Study selection
We found a total of 963 potentially eligible articles, 666 of which were obtained after removing duplicate articles. Subsequently, 601 non-pertinent articles were excluded by evaluating titles and abstracts. In the remaining 65 articles, the full texts were accessed. Ultimately, 11 articles [28][29][30][31][32][33][34][35][36][37][38] were included in the present analysis, including a total of 878 patients where the predictive values of postoperative neuronal injury biomarkers were compared in the POCD and non-POCD groups (Fig 1).

Study characteristics
The demographic and clinical characteristics of each study are presented in Table 1. The sample sizes of the studies varied greatly (POCD groups, 22.36 ± 11.22; non-POCD groups, 57.45 ± 42.79). The median Newcastle-Ottawa Scale score was 7.6 (S3 Appendix), indicating that the quality of these articles was high. The bias risk assessment suggested that there was little evidence of significant bias in the included RCT (S4 Appendix). The number of studies on tau (n = 1) and NFL (n = 1) was insufficient for meta-analysis, and a descriptive summary of the results was performed. GFAP studies were not included because the data were not available. Therefore, three biomarkers, S100β, NSE, and Aβ, were quantitatively analyzed in the present study.

Results of individual studies
Tau and NFL biomarkers. In our included studies, only Evered et al. [31] measured total tau, phosphorylated tau, and NFL biomarkers in the CSF. Because the number of studies was insufficient, a quantitative analysis could not be performed. Qualitative analysis showed that there were no statistically significant differences in these three biomarkers between the POCD and non-POCD groups 7 days or 3 months postoperatively. S100β, NSE, and Aβ biomarkers. According to the first postoperative sampling timepoint, the observational studies included showed that S100β was significantly higher in patients with POCD than in those without POCD (SMD: 4.14, 95% CI: 1.91−6.38). No significant differences in NSE (SMD: 0.58, 95% CI: -0.12−1.29) and Aβ (SMD: 1.43, 95% CI: -0.24 −3.11) were detected between the two groups (Fig 2A). The included RCT showed that S100β (SMD: 37.31, 95% CI: 30.97−43.64) and NSE (SMD: 3.50, 95% CI: 2.71−4.28) were significantly higher in POCD patients than in non-POCD patients (Fig 2B).

Sensitivity analysis
Sensitivity analysis was used to explore the impact of each study on the overall estimation. The results of the sensitivity analysis showed that when the study by He et al. [35] was removed, there was no significant difference in S100β level between POCD patients and non-POCD patients (P = 0.25). When the study by Linstedt et al. [28] was removed, the NSE was significantly higher in patients with POCD than in those without POCD (P <0.0001). When the study by Evered et al. [31] was removed, Aβ levels were significantly higher in patients with POCD than in those without POCD (P <0.00001). This suggests that the results summarized at the first postoperative sampling should be interpreted with caution.

Publication bias
The Egger test did not show evidence of publication bias (S5 Appendix).

Discussion
Ntalouka et al. [39] showed that orthopedic and cardiac surgeries were associated with a higher incidence of POCD. In this meta-analysis, cardiac surgery [35][36][37][38] and hip replacement surgery [29,31] represented more than half of the total included studies. The incidence of POCD in the included studies ranged from 16.66% to 56.36%, with the highest incidence being openheart surgery [37], which may further strengthen the association between biomarkers and POCD. S100β is a 21 kDa protein with a homologous dimer structure that belongs to the calciummediated protein family [40]. The protein may be involved in the growth, proliferation, and activation of neurons and glial cells and can be expressed and secreted in the CSF through astrocytes after a central nervous system injury [14]. The present analysis showed that S100β was associated with POCD (Fig 5), which is consistent with a previous finding [41]. However, some studies suggested that S100β was not related to postoperative cognitive results [8,42,43]. The pooled data of the included observational studies showed that S100β at 1 hour, 2 days, and 9 days postoperatively was associated with POCD, while S100β at 6 hours and 24 hours postoperatively was not significantly correlated with POCD. Further studies are warranted to confirm this finding. Consistent with the results of the observational studies, the RCT showed that S100β was associated with POCD at 2 days and 9 days postoperatively.
NSE is a dimer enzyme in neurons and neuroendocrine cells and has a half-life of 24 hours [14]. NSE has been confirmed to provide quantitative measures of brain injury and improve the diagnosis and outcome evaluation of many brain-related diseases, such as ischemic stroke, intracerebral hemorrhage, seizures, and traumatic brain injury [44]. Although observational studies based on first postoperative sampling time showed no significant correlation between NSE and POCD, the pooled data showed that NSE was significantly correlated with POCD at 1 hour, 6 hours, 24 hours, 2 days, and 9 days postoperatively. The inconsistency of the results may be due to the different sampling times of the markers. In the included studies, Linstedt et al. [28] reported that the first sampling time of markers was 30 minutes postoperatively, while other studies were at 1 hour to 2 days postoperatively. Sensitivity analysis showed that when the study by Linstedt et al. [28] was removed, NSE was significantly correlated with POCD (P<0.0001).
Aβ is derived from the proteolysis of the β-amyloid precursor protein [45]. Progressive accumulation of Aβ is a pathological feature of AD. The accumulation of Aβ in synaptic mitochondria can damage neuronal function and is related to neuronal degeneration [45,46]. In the present analysis, all of the studies involving the Aβ biomarker were observational studies, and the results showed that Aβ was not significantly associated with POCD according to the earliest postoperative sampling time-point. However, the pooled data showed that Aβ levels were significantly correlated with POCD at 24 hours, 2 days, and 9 days postoperatively. This may be due to the fact that the first sampling time of markers reported by Evered et al. [31] was 7 days postoperatively, which was significantly longer than that reported by other studies

PLOS ONE
that sampled Aβ from 1 hour to 2 days postoperatively. In addition, the biomarkers used in this study were derived from the CSF, while the biomarkers of other studies were derived from blood. In particular, the Aβ value in this study was lower in the POCD group than in the non-POCD group, which differed from the conclusions of other studies that the biomarkers in the POCD group were higher than in the non-POCD group. The sensitivity analysis also showed that when the study by Evered et al. [31] was removed, Aβ was significantly correlated with POCD (P <0.00001).
Tau is a microtubule-associated protein in the brain and spinal cord that can stabilize axon microtubules [14]. In neurodegenerative diseases, such as AD, tau protein phosphorylation is related to nerve death [47]. Among the included studies, only one detected this biomarker; therefore, a pooled analysis of tau could not be performed. However, according to the results of Evered et al. [31] and a previous finding [21], tau is not significantly associated with POCD.
Neurofilaments are abundant structural scaffolding proteins that are expressed exclusively in neurons. Neurofilaments have attracted increasing attention as candidate biomarkers of axonal injury. NFL, a neurofilament, is considered the most promising biomarker for neurological diseases [48]. Similarly, only Evered et al. [31] detected this biomarker in the included studies. According to their results [21] and those of others [49], NFL has a limited predictive value for POCD.
GFAP is an important component of the astrocyte cytoskeleton and is associated with cell regeneration, synaptic plasticity, and reactive glial cell proliferation [50]. However, none of the included studies detected this marker. But, previous studies [21,43,51,52] have shown that GFAP was not significantly associated with POCD.
In particular, two studies have different conclusions about whether the ratio of Aβ and tau is associated with postoperative neurocognitive decline. Berger et al. [53] showed that neurocognitive decline after noncardiac and non-neurosurgical operations is unlikely to be associated with changes in tau/Aβ ratios. However, Wu et al. [54] showed that the Aβ-42/tau ratio can be used to predict the development of POCD in the elderly. Therefore, further studies are required to address this issue.
In 2018, relevant experts presented recommendations [55] for the nomenclature of cognitive changes associated with anesthesia and surgery. Before renaming, changes in postoperative cognitive function were divided into delirium during the recovery period, postoperative delirium, and POCD. Similar to POCD, the relationship between postoperative delirium and neuronal injury biomarkers remains unclear. Ballweg et al. [56] have shown that changes in plasma tau protein are associated with the incidence and severity of delirium. Three studies [57][58][59] found that patients with elevated NFL levels were more prone to delirium. However, two studies [60, 61] found that postoperative delirium was not associated with Aβ1-42, tau, and S100β levels. According to the recommendations [55], using the term "neurocognitive disorders" to describe changes in cognitive function from the preoperative period to 12 months postoperatively will help reduce the impact caused by the different evaluation times of cognitive function.
To our knowledge, this is the first meta-analysis to explore the predictive value of neuronal injury biomarkers for POCD. The results of our analysis showed that the three biomarkers, S100β, NSE, and Aβ, have predictive value for POCD. The relationship between biomarkers and postoperative cognitive outcomes is closely related to their detection time.
This meta-analysis had some limitations. Firstly, heterogeneity was high in many analyses. Heterogeneity may be due to the following reasons: (1) small sample size of each study, (2) different sources (blood or CSF), detection time, and biomarker identification methods, (3) different evaluation times and diagnostic methods of POCD, (4) different types of surgery, and (5) patients of different ages. Secondly, the number of articles included was relatively small and most of the analyses were based on a few studies, which hindered us from conducting a more in-depth analysis of patient age, surgical type, anesthesia method, and source of marker samples. Thirdly, due to the lack of available data, we could not further analyze the impact of preoperative biomarker levels on the present findings. Fourthly, due to the inconsistent diagnosis time of POCD, the predictive value of biomarkers for postoperative short-and longterm cognitive function could not be compared. Therefore, future studies should focus on these issues.
Biomarkers derived from CSF are considered to be the gold standard for POCD-related biomarkers. However, due to the difficulty of extracting CSF samples from patients taking anticoagulant drugs or emergency patients, their clinical application is limited [15]. This means that blood samples may be an easier source than CSF. Among the studies included in this analysis, only one study [31] sample was derived from CSF, which may also be explained by the above reasons. Cata et al. [14] proposed that the S100β level at 24 hours after surgery has high sensitivity and specificity in identifying patients with brain injury; tau protein levels peaked at 6 hours after surgery and returned to baseline levels on the fourth day after surgery. The study by Herrmann et al. [62] showed that concentrations of NSE returned to baseline at 24 hours after surgery. However, our meta-analysis showed no significant correlation between S100β and POCD at 6 hours and 24 hours after surgery. Future studies are necessary to optimize the sampling time point for the better prediction of POCD. According to the recommendations by Evered et al. [55], the objective criteria of POCD should be based on one or more cognitive domains, including attention, executive function, learning and memory, language, perceptual movement, or social cognition. Compared to screening tools, such as the Minimental State Examination or Montreal Cognitive Assessment, using psychometric assessments can objectively assess specific cognitive domains. And the time to evaluate cognitive function should be within 30 days to 12 months after surgery. Future studies are warranted to unify the source of biomarkers, sampling time points, and diagnostic methods of POCD so as to further demonstrate the predictive value of biomarkers in perioperative cognitive function.
Early identification of patients with high risk of POCD helps facilitate preventive measures to reduce the incidence of disease, accelerate postoperative rehabilitation, shorten hospitalization time, and reduce medical costs. Early enteral feeding, postoperative multimodal analgesia, increased physical activity, and social participation, as well as care for patients in a quiet environment with the presence of family members, can help improve the cognitive function of the elderly. However, the therapeutic effect of related drugs on POCD, including memantine, huperzine A, and brain-derived neurotrophic factor, remains to be further confirmed [1].
This meta-analysis identified that postoperative S100β, NSE, and Aβ levels have predictive value for POCD. The relationship between these biomarkers and POCD may be affected by sampling time. Early diagnosis and prevention of POCD are important in postoperative clinical practice, and future studies with larger sample sizes are warranted to confirm this finding.